1. Field of the Invention
This invention relates to ignition timing control apparatus for an internal combustion engine, or more particularly to ignition timing control apparatus having means retarding ignition timing so as to decrease toxic components, especially nitrogen oxides and hydrocarbons, contained in exhaust gas.
2. Description of the Prior Art
In ignition timing control apparatus of conventional types, retardation of ignition timing has been effected by cutting off the vacuum introduction into the vacuum advancer from a carburetor, during ordinary operation conditions, that is, other than during the warming-up and cold operations.
On the other hand, it is desirable to change the degree of ignition timing retardation with the level of vacuum from the suction pipe, because the reaction rate of the fuel/air mixture generally changes with the vacuum.
However, the conventional retarding mechanism, established by only cutting off the introduction of vacuum into the vacuum advancer from the carburetor, does not change the degree of retardation in accordance with changing vacuum. Therefore, if such apparatus is so designed as to provide the best suited retardation under high load condition, not optimum but excessive retardation will result under medium load conditions.